-I have a dell Inspiron 1520 with Broadcom Broadcom 440x 10/100 Integrated Controller, would it do any good to upgrade to a gigabit capable NIC?
-I am also using a D-Link DIR-655 Wifi router, think its gigabit...
??I'd also need to make sure all Ethernet cables are cat6, right (vs cat5)?
??Is the other hardware, in laptop, going to bottleneck/seriously restrict bandwidth making the gigabit upgrade less than a good investment?
You won't be able to upgrade the internal card, you need to buy an external gig ethernet card. Laptop will use the faster speeds, but as was pointed out already, that's only good if every other system you talk to runs at gigabit speeds.